Abstract

We studied the transport of a nonreactive tracer in an unconsolidated weathered granite column under steady-state flow and transient flow induced by water-table fluctuation in the bottom half of the column. Experiments reproducing one-dimensional vertical flow and tracer transport were performed at two infiltration rates, 12 and 22 mm h−1. Breakthrough data were used (i) to compare tracer dispersion under steady-state and transient flow, (ii) to analyze the mechanisms responsible for tracer transport under both flow conditions, and (iii) to test the ability of existing transport models to reproduce observed breakthrough curves (BTCs). The BTCs under steady-state flow are typical of physical nonequilibrium transport in a dual-porosity medium. The shapes of BTCs under transient flow are less common, showing multiple peaks. The spreading of the tracer was one order of magnitude greater under transient flow than under steady-state flow. We attribute the multiple-peak shape of BTCs and the greater spreading under transient flow to rapid convection in a rapid mobile domain. Diffusion and advection could account for tracer exchanges between the slow mobile domain, which is active during unsaturated water flow, and the rapid mobile domain, which becomes active as soon as the pore space is fully saturated. Water flow was simulated with the Richards equation using HYDRUS-1D. A mobile–immobile transport model coupled to the Richards equation reproduced the BTCs satisfactorily under steady-state flow at both infiltration rates; however, it was unable to reproduce the multiple peaks of tracer concentrationse under a fluctuating water-table regime.

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